Droplet ejection devices are used for a variety of purposes, most commonly for printing images on various media. They are often referred to as ink jets or ink jet printers. Drop-on-demand droplet ejection devices are used in many applications because of their flexibility and economy. Drop-on-demand devices eject a single droplet in response to a specific signal, usually an electrical waveform, or waveform.
Droplet ejection devices typically include a fluid path from a fluid supply to a nozzle path. The nozzle path terminates in a nozzle opening from which drops are ejected. Droplet ejection is controlled by pressurizing fluid in the fluid path with an actuator, which may be, for example, a piezoelectric deflector, a thermal bubble jet generator, or an electrostatically deflected element. A typical printhead has an array of fluid paths with corresponding nozzle openings and associated actuators, and droplet ejection from each nozzle opening can be independently controlled. In a drop-on-demand printhead, each actuator is fired to selectively eject a droplet at a specific target pixel location as the printhead and a substrate are moved relative to one another. In high performance printheads, the nozzle openings typically have a diameter of 50 micron or less, e.g., around 25 microns, are separated at a pitch of 100-300 nozzles/inch, have a resolution of 100 to 300 dpi or more, and provide droplet sizes of about 1 to 100 picoliters (pl) or less. Droplet ejection frequency is typically 10-100 kHz or more but may be lower for some applications.
Hoisington et al. U.S. Pat. No. 5,265,315, the entire contents of which is hereby incorporated by reference, describes a printhead that has a semiconductor printhead body and a piezoelectric actuator. The printhead body is made of silicon, which is etched to define fluid chambers. Nozzle openings are defined by a separate nozzle plate, which is attached to the silicon body. The piezoelectric actuator has a layer of piezoelectric material, which changes geometry, or bends, in response to an applied voltage. The bending of the piezoelectric layer pressurizes ink in a pumping chamber located along the ink path. Deposition accuracy is influenced by a number of factors, including the size and velocity uniformity of drops ejected by the nozzles in the head and among multiple heads in a device. The droplet size and droplet velocity uniformity are in turn influenced by factors such as the dimensional uniformity of the ink paths, acoustic interference effects, contamination in the ink flow paths, and the actuation uniformity of the actuators.
Because drop-on-demand ejectors are often operated with either a moving target or a moving ejector, variations in droplet velocity lead to variations in position of drops on the media. These variations can degrade image quality in imaging applications and can degrade system performance in other applications. Variations in droplet volume lead to variations in spot size in images, or degradation in performance in other applications. For these reasons, it is usually preferable for droplet velocity, droplet volume and droplet formation characteristics to be as constant as possible throughout the operating range of an ejector.
Droplet ejector producers apply various techniques to improve frequency response, however, the physical requirements of firing drops in drop-on-demand ejectors may limit the extent to which frequency response can be improved. “Frequency response” refers to the characteristic behavior of the ejector determined by inherent physical properties that determine ejector performance over a range of droplet ejection frequencies. Typically, droplet velocity, droplet mass and droplet volume vary as a function of frequency of operation; often, droplet formation is also affected. Typical approaches to frequency response improvement may include reducing the length of the flow passages in the ejectors to increase the resonant frequency, increase in fluidic resistance of the flow passages to increase damping, and impedance tuning of internal elements such as nozzles and restrictors.